Colour reversal film produces transparencies that can be mounted as slides and projected or viewed with a slide viewer or on a light box. Transparencies can also be made into prints, either directly, or from what is called an inter negative, which involves photographing the transparency onto the other kind of film, colour negative film.
Colour negative film is the type used to make prints.

Most professional photographers use colour reversal film, whereas most amateurs prefer colour negative.

Both types of film come in a full range of sizes, from the small 110 up through 35 mm to 10 X 8 in (30 X 20 cm) sheet film. They also come in various speeds. These are given as an ISO (International Standards Organization) number from about 25 for the slowest up to 1600 and beyond for the fastest. The slower the film, the finer the grain and sharpness, and the greater the colour saturation and contrast. 1600 ISO film can be uprated to 3200 ISO and more for work in low light but the result will be very grainy — an effect which may be sought deliberately.

Uprating Film and DX Coding

Uprating, also called speed readjustment, means using film as if it had a higher ISO rating than it actually does, and so shortening exposure time. Uprated film needs a longer development time, and if you uprate any film you must let the laboratory know that you have done this so that they can ‘push’, or extend, development. Some laboratories charge extra for handling uprated film. Generally only colour reversal film is uprated since the processing for all colour negative films is the same. However, a few colour negative films are now made in such a way that they can be uprated.

35 mm film is now DX coded: this means that the film cassette has a bar code on it. Nearly all new cameras have tiny sensors that read the code and automatically change the camera’s ISO setting to the appropriate speed. If you want to uprate the film you should buy bar code override stickers to put on the cassette. Alternatively, if your camera has an exposure compensating dial, you can adjust this to uprate the film. In either case you must inform the laboratory of what you have done.